1. Field of the Invention
The present invention relates to an optical transmission apparatus and an optical transmission method, and more particularly to an apparatus and a method for optical transmission including a frequency-shift-keying (‘FSK’) scheme.
2. Description of the Related Art
FSK schemes are shift-keying schemes that load information on the frequency of an optical signal. FSK schemes have a higher reception sensitivity than that of the conventional on-off keying schemes (e.g., by about 3 dB). FSK schemes also allow for strong fiber nonlinearity. Since the information is included in the frequency of an optical signal other than the intensity of the optical signal, FSK schemes are rarely affected by the kerr nonlinearity of an optical fiber. Moreover, since the carrier frequency of a signal is suppressed, FSK schemes are strong against a Brillouin nonlinearity effect. FSK schemes can also accept strong optical fiber nonlinearity as compared with PSK schemes (phase-shift-keying). Since this means that a signal can be transmitted over a long distance without signal reproduction, a long distance optical transmission system can be more effectively achieved by means of such a characteristic.
In order to generate an FSK signal, it is necessary to provide an apparatus capable of generating different frequencies according to electric data signals. The conventional method for generating the FSK signal uses the chirping of a laser diode. Since an optical signal output from the laser diode has a frequency changed according to electric current applied to the laser diode, the FSK signal can be generated by means of the above characteristic of the laser diode.
FIG. 1 is a circuit diagram showing an example of a conventional optical transmission apparatus 100 employing a typical FSK scheme. The optical transmission apparatus 100 includes a distributed feedback laser (DFB) 110 and a bias-tee circuit 120.
The distributed feedback laser 110 generates two tone signals having different frequencies according to input data signals. When ‘0’ bit data is input, the distributed feedback laser 110 generates a first tone signal. When ‘1’ bit data is input, the distributed feedback laser 110 generates a second tone signal. This is because the distributed feedback laser 110 has a changed output frequency and intensity according to the intensity of input data. Further, in addition to the data signal, a bias signal having a constant electric current is input to the distributed feedback laser 110. When such a direct modulation scheme is employed, both the output frequency and the output intensity of the distributed feedback laser 110 are modulated and an output signal is distorted.
The bias-tee circuit 120 includes an inductor L 130 disposed between a bias terminal and the distributed feedback laser 110 in order to block alternating current. A condenser C 140 is disposed between a data terminal and the distributed feedback laser 110 in order to block direct current.
FIG. 2 is a circuit diagram of the second example of a conventional optical transmission apparatus 200 employing a typical FSK scheme.
The optical transmission apparatus 200 includes a distributed feedback laser 210, an intensity modulator MOD 260, an inverting amplifier 250 and a bias-tee circuit 220.
The distributed feedback laser 210 may generate two tone signals having different frequencies according to input data signals. When ‘0’ bit data is input, the distributed feedback laser 210 generates a first tone signal. When ‘1’ bit data is input, the distributed feedback laser 210 generates a second tone signal. In addition to the data signal, a bias signal having a constant electric current is input to the distributed feedback laser 210.
The inverting amplifier 250 inverts the input data signal to output the inverted data signal to the intensity modulator 260.
The intensity modulator 260 changes the intensity of the tone signal input from the distributed feedback laser 210 according to the input inverted data signal. The intensity modulator 260 offsets the distortion of an output signal due to intensity change of the data signal, which is input to the distributed feedback laser 210, by means of the inverted data signal. Therefore, the output signal due of the intensity modulator 260 has a constant intensity regardless of the frequency of the output signal.
The bias-tee circuit 220 includes an inductor L 230 disposed between a bias terminal and the distributed feedback laser 210 in order to block alternating current. A condenser C 240 is disposed between a data terminal and the distributed feedback laser 210 in order to block direct current.
The optical transmission apparatus 200 has a reduced intensity distortion of an output signal as compared with the optical transmission apparatus 100. However, since the optical transmission apparatus 200 also employs a direct modulation scheme, the optical transmission apparatus 200 has a limitation in a modulation speed. Typically, a modulation bandwidth of a laser is determined by a relaxation oscillation frequency of the laser. The relaxation oscillation frequency increases by the square root of a bias current of the laser, but is generally less than 20 GHz. Accordingly, it is difficult to apply the relaxation oscillation frequency to a high speed transmission system of more than 40 Gbps. Further, since an optical frequency generated by the chirping of a laser has a phase somewhat different from that of an applied current, it is difficult to generate a clean FSK signal having a small chirping from the optical frequency.